EP0653237A1 - Process for reducing the concentration of nitrogen oxides in the exhaust gas of combustion engines or incinerators - Google Patents

Abstract

A significant reduction in the NOx emission of a diesel engine (1) can be achieved by using the SCR method. In this process, ammonia (6) is injected into a catalytic converter (2) through which the exhaust gas flows, where it reacts with nitrogen monoxide or nitrogen dioxide to give nitrogen and water. Since the exhaust gas should not contain nitrogen monoxide or excess ammonia, suitable methods for regulating (8) the NH3 dosage (5) are required. To regulate (8) the amount of urea added to the exhaust gas as a reducing agent, the NO and NH3 concentration is measured with the aid of a detector (9) arranged in the exhaust tract behind the SCR catalytic converter. As a sensitive element, the detector (9) contains a vanadate layer produced by a special sputtering process. Their electrical resistance is greatest when the conversion of nitrogen monoxide to nitrogen and water is stoichiometric. <IMAGE>

Description

The nitrogen oxide and particle emissions (dust) of a diesel engine optimized for performance and consumption can only be reduced insignificantly by means of combustion technology. In order to be able to comply with the exhaust gas limit values prescribed by law in the future, aftertreatment of the diesel engine exhaust gases is therefore essential.

A significant reduction in NO _x emissions from a diesel engine can be achieved by application of the so-called S elective- C atalytic- R eduction technique. In the SCR process, gaseous ammonia NH₃, ammonia in aqueous solution or urea is injected as a reducing agent in the exhaust system, so that in particular the chemical reactions on a catalyst



4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O




2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O



can expire. To reduce 1 mole of NO _x in the diesel engine exhaust gas requires about 0.9 to 1.1 moles of NH₃. If less ammonia NH3 is injected, the catalyst no longer works with the highest efficiency. Overdosing should also be avoided, as otherwise unused ammonia NH3 will get into the atmosphere.

The SCR process known from DE 36 10 364 can reduce the proportion of NOx in the exhaust gas from combustion plants by more than 80% and at the same time limit the NH 3 emissions to below 5 ppm. The dosage of the reducing agent is monitored Computer that evaluates the output signal of an NH 3 sensor arranged in the exhaust tract behind the NO _x converter and, if necessary, readjustes the supply of reducing agent by controlling a delivery unit. An NH₃ sensor is an electrochemical cell, which contains a cup-shaped body made of stabilized zirconium dioxide as an essential component. Two electrodes are applied to the solid electrolyte, the outer electrode exposed to the exhaust gas being made, for example, of TiO₂, Pt V₂O₅ or V₂O₅ and the inner electrode exposed to a reference gas (air) made of platinum.

The process described in DE 36 06 635 for reducing the NO _x concentration uses the high temperatures of the exhaust gas to evaporate the reducing agent present in solid or liquid form and to split it into reactive components. After concentration, the reactive components are injected via a bundle of pipes into a relatively cool zone of the exhaust tract, where they react with the nitrogen oxides to nitrogen, water and carbon dioxide. A sensor working according to the chemiluminescence comparison method is used to measure the NO _x concentration. Its output signal is fed to a controller with setpoint specification, which controls the servomotor of a metering valve.

The aim of the invention is to provide a method with which the concentration of nitrogen oxides NO _x in the exhaust gas of an internal combustion engine or an incinerator can be significantly reduced. In particular, it should be ensured that the exhaust gas contains neither nitrogen monoxide NO nor excess ammonia NH₃. According to the invention, these objects are achieved by a method according to claim 1.

The advantage that can be achieved with the invention is, in particular, that the NH₃ amount required for a stoichiometric conversion of nitrogen monoxide NO to nitrogen N₂ and water H₂O in a simple manner by measuring the electrical resistance of a vanadate layer exposed to the exhaust gas can determine. An absolute measurement of the NO or NH₃ concentration is not necessary. Since the aim of the control is a maximum resistance of the metal oxide layer used as the sensor element, any resistance drift that may be present does not cause any problems.

Fig. 1

eine mit einem SCR-Katalysator ausgestattete Abgasanlage eines Dieselmotors

Fig. 2 und 3

den schematischen Aufbau eines NO/NH₃-Detektors

Fig. 4

die Kammelektroden des NO/NH₃-Detektors

Fig. 5

Verfahrensschritte zur Herstellung der Kammelektroden

Fig. 6

eine auf den Kammelektroden abgeschiedene Al₂O₃-V₂O₅-Sandwich-Struktur

Fig. 7 bis 11

die Sensitivität einer AlVO₄-Dünnschicht auf Stickstoffmonoxid NO, Ammoniak NH₃ und andere Gase

Fig. 12

ein Ausführungsbeispiel eines Detektorgehäuses

Fig. 13

ein das Verfahren zur Regelung der eingespritzen Harnstoffmenge erläuternden Ablaufplan

Fig. 14

den elektrischen Widerstand einer Vanadatschicht in einem Stickstoffmonoxid NO und Ammoniak NH₃ enthaltenden Gasgemisch in Abhängigkeit von der Menge des dem Gasgemisch zugesetzten Ammoniaks NH₃

The dependent claims relate to advantageous developments and refinements of the invention explained below with reference to the drawings. Here shows:

Fig. 1

an exhaust system of a diesel engine equipped with an SCR catalytic converter

2 and 3

the schematic structure of a NO / NH₃ detector

Fig. 4

the comb electrodes of the NO / NH₃ detector

Fig. 5

Process steps for the production of the comb electrodes

Fig. 6

an Al₂O₃-V₂O₅ sandwich structure deposited on the comb electrodes

7 to 11

the sensitivity of an AlVO₄ thin film on nitrogen monoxide NO, ammonia NH₃ and other gases

Fig. 12

an embodiment of a detector housing

Fig. 13

a flow chart explaining the process for controlling the amount of urea injected

Fig. 14

the electrical resistance of a vanadate layer in a nitrogen monoxide NO and ammonia NH₃ containing gas mixture depending on the amount of ammonia NH₃ added to the gas mixture

The exhaust system of a diesel engine 1 shown schematically in FIG. 1 is said to largely break down the nitrogen oxides NOx formed during operation and to release the remaining residual gases to the atmosphere with as little noise as possible. It consists of an SCR catalytic converter 2 described in / 1 / and / 2 /, one or more switching dampers 3 and a pipe system 4 which connects the individual components to the exhaust gas outlet openings present in the cylinder head of the diesel engine 1. Upstream of the SCR catalytic converter 2 is a metering device 5, which is stored in a storage container 6 Injecting reducing agent into the exhaust tract. The metering device 5 contains in particular a membrane pump connected to an injection nozzle 7 or an injection valve with an upstream flow meter. A control unit 8 ensures that a certain amount of the ammonia-containing reducing agent can be supplied to the exhaust gas.

A particularly suitable reducing agent is an aqueous urea solution (CO (NH₂) ₂). This is decomposed with the addition of heat to carbon dioxide CO₂ and ammonia NH₃, the ammonia NH₃ adsorbed on the surface of the catalyst 2 and reacts with the nitrogen oxides NO and NO₂ present in the exhaust gas to form the non-toxic substances nitrogen N₂ and water H₂O. To ensure that the diesel exhaust gas entering the environment contains neither nitrogen monoxide NO nor excess ammonia NH₃, the reaction must



4NH₃ + 4NO + O₂ → 4N₂ + 6H₂O



run stoichiometrically. According to the invention, the NO or NH₃ concentration is therefore measured with the aid of a detector 9 arranged in the exhaust pipe 4 behind the SCR catalytic converter 2 and used to control the amount of urea injected. The amount of urea required for a stoichiometric reaction is then injected due to the properties of the detector to be described when the electrical resistance of a vanadate layer used as an NO- or NH₃-sensitive element passes through a maximum or its electrical conductivity passes through a minimum.

The substrate 10 of the detector 9 shown in Figures 1 and 2 consists of an electrically insulating material such as glass, beryllium oxide BeO, aluminum oxide Al₂O₃ or silicon (with Si₃N₄ / SiO₂ insulation). On the between 0.1 and 2 mm thick substrate 10 are two platinum electrodes 11, 11 'forming an interdigital structure, a vanadate layer 12 (AlVO₄ or FeVO₄) connecting these electrodes as NH₃- or NO-sensitive element and a temperature sensor 13 arranged. The passivation layer made of silicon oxide, designated 14, shields the connecting leads 15, 15 'and 16, 16' respectively assigned to the two comb electrodes 11, 11 'and the temperature sensor 13 from the oxygen present in the exhaust gas.

In order to be able to set the desired operating temperature of up to 600 ° C. and to be able to keep it constant regardless of external influences, the detector 9 is actively heated with the aid of a resistance layer arranged on the rear side of the substrate 10. The resistance layer designated 17 in FIG. 2 consists, for example, of platinum (Pt), gold (Au) or an electrically conductive ceramic and has a meandering structure. Also shown is the approximately 10 to 100 nm thick metal layer 18 consisting of titanium (Ti), chromium (Cr), nickel (Ni) or tungsten (W), which improves the adhesion between the substrate 10 and the platinum electrodes 11, 11 ' .

The dimensions of the comb electrodes 11 and 11 'depend on the specific resistance of the sensor layer 12 applied over it in the desired temperature range. For example, the comb structure 11, 11 ′ can have thicknesses of 0.1 to 10 μm, widths of 1 to 1000 μm and electrode spacings of 1 to 100 μm. For a 1 µm thick AlVO₄ layer 12, the following dimensions lead to well-measurable specific resistances in the temperature range between 500 and 600 ° C: electrode thickness D = 1.5 µm, length of the interdigital structure L = 1 mm, electrode spacing S = 50 µm.

FIG. 4 shows a scale representation of an interdigital structure in a top view. In this embodiment, a platinum resistance layer 19 is used as the temperature sensor. To produce the comb electrodes 11, 11 ', a 1.5 μm thick platinum layer 20 is first deposited on the heated corundum substrate 10 in a sputtering system (see FIGS. 5a, b). The structuring of the layer 20 takes place in a positive photo step, in which the photoresist 21 is applied at the location of the electrodes to be produced and exposed through a mask 22 (see FIG. 5c, d, e). The developed photoresist 21 protects the platinum layer 20 during the subsequent etching step (see FIG. 5f). After removing the photoresist 21 with acetone, the desired comb electrodes 11 and 11 '(see FIG. 5g) are obtained, on which the gas-sensitive vanadate layer 12 is subsequently deposited (see FIG. 5h).

The use of gold Au instead of platinum Pt as the electrode material has no influence on the gas sensitivity of the Al₂O₃ / V₂O₅ mixed oxide.

The extraordinary properties of the detector are based on the sputtering method to be used in the production of the gas-sensitive layer 12 and the subsequent tempering. The comb electrodes 11, 11 'can be coated, for example, in the Leybold Z490 sputtering system. Metallic vanadium (V) and aluminum (Al) serve as starting materials, which are reactively atomized from corresponding targets in a plasma consisting of 80% argon and 20% oxygen and are deposited on the heated substrate 10. The sandwich structure 23 shown in FIG. 6 is built up by alternately atomizing the two targets. It has a thickness of approximately 1 µm and consists of 60 to 80 approximately 10 to 15 nm thick V₂O₅ or Al₂O₃ layers, the Al₂O₃ content being 50% to a maximum of 70%. The sputtering parameters are given in the following table. Residual gas pressure approx. 2 - 4 x 10⁻⁶ mbar Sputtering gas pressure 4.2 x 10⁻³ mbar Sputtering gas 20% O₂ / 80% Ar DC potential Al target: 155V V target: 225 V. Substrate temperature approx. 250 ° C

In order to produce a homogeneous mixed oxide, the sandwich structure 23 is annealed in air in a high-temperature furnace for about 5 to 15 hours. The furnace temperature has a decisive influence on the topography and the phase of the Al₂O₃ / V₂O₅ layers. An optimal sensitivity for ammonia NH₃ and nitrogen monoxide NO show layers that were annealed at temperatures T between 550 ° C ≦ T ≦ 610 ° C and consist of equal proportions of V₂O₅ and Al₂O₃. Tempering creates the aluminum vanadate AlVO₄, which is responsible for the high gas sensitivity. The maximum working temperature of the vanadate layer is around 600 ° C. Aluminum vanadate AlVO₄ has a triclinic unit cell with a = 0.6471 nm, b = 0.7742 nm, c = 0.9084 nm, α = 96.848 A, β = 105.825 A and χ = 101.399 A, the volume of which is V = 0.4219 nm³.

Layers with an Al₂O₃ content of more than 50% show a somewhat smaller measurement effect. However, they can still be used at higher temperatures of up to 680 ° C.

The following diagrams are intended to document the sensitivity or sensitivity of the AlVO₄ thin layers produced by the described method to different gases. The size σ / σ _o (σ _o : conductivity of the sensitive layer in synthetic air (80% N₂ / 20% O₂)) is plotted as a function of the time t and the concentration of the respective gas.

Even the presence of the smallest amounts of nitrogen monoxide NO and ammonia NH₃ in dry synthetic air leads to a significant increase in the conductivity of the aluminum vanadate AlVO₄ (see Fig. 7 and 8). The conductivity changes by about 75% if you add 10 ppm nitrogen monoxide NO to the air. The addition of 10 ppm ammonia NH₃ has an increase in conductivity by more than a factor of 6.

9 shows, the specific resistance of the AlVO₄ thin film increases in the presence of nitrogen dioxide NO₂. Since the aluminum vanadate shows a completely different behavior compared to nitrogen monoxide NO (reduction of the specific resistance, see Fig. 7), one can clearly differentiate between the two nitrogen oxides, provided only one of the two nitrogen oxides interacts with the sensitive element.

In addition to nitrogen monoxide NO and ammonia NH₃, the vanadate layer also responds to changes in the oxygen partial pressure and hydrogen H₂ (see FIG. 10). The cross sensitivity to oxygen O₂ and hydrogen H₂ is considerably smaller than the reaction to nitrogen monoxide NO and ammonia NH₃. For example, 500 ppm of hydrogen H₂ in air result in approximately the same change in conductivity as the addition of 10 ppm of nitrogen monoxide NO. The gases carbon monoxide CO (up to 1500 ppm), methane CH₄ (up to 5000 ppm) and carbon dioxide CO₂ (up to 1%) are undetectable up to the concentrations given in brackets. In a moist gas mixture (80 mbar H₂O) a significant decrease in the NH₃ sensitivity is observed; however, it still remains twice as sensitive to nitrogen monoxide NO (see the right part of FIG. 9).

In Fig. 11, the sensitivity of the AlVO₄ thin film in moist air (80 mbar H₂O) is shown at 500 ° C and a NO portion of 10 ppm. Another gas in the specified concentration was added to the moist air within the time intervals marked by a horizontal line. The air therefore contained, for example, 1500 ppm carbon monoxide CO between the 60th and the 120th minute and an additional 10 ppm of nitrogen monoxide NO between the 80th and the 100th minute. As the measurement results show, the NO sensitivity of the AlVO₄ layer is not affected by the presence of carbon monoxide CO, methane CH₄ and carbon dioxide CO₂. The addition of hydrogen H₂ causes no masking of the NO sensitivity, but a clear cross sensitivity can be determined. A similar effect is observed with oxygen O₂ when its concentration decreases from 20% to 2%.

The stainless steel housing shown in FIG. 12 is used to install the detector 9 in the wall of the exhaust pipe 4. The housing consists of two parts, the housing head 26 having a gas inlet opening 24 and a metal web 25 on the one with a bore 27 for receiving the detector 9 provided base body 29 is attached. Before welding the two parts 26 and 29, the detector 9 is glued in the bore 27 of the base body 29. After assembly, the sensitive element is located in an S-shaped flow channel which connects the gas inlet opening 24 to the gas outlet opening 30. In the left part of FIG. 12, the ceramic plate 31 closing the bore 27 of the lower housing part 29 is also shown. It contains several channels through which the connecting wires 32 used for contacting the detector 9 are led to the outside.

With the help of the NO / NH₃ detector 9 described above, a simple and effective method for controlling the urea injection into the SCR catalyst 2 can be realized. Since the detector 9 reacts to both nitrogen monoxide NO and ammonia NH₃ with an increase in conductivity (see FIGS. 7 and 8), it is initially not possible to decide which of the two gases interacts with the sensitive layer 12. As can be seen from the flow chart shown in FIG. 13, the control unit 8 will cause the metering device 5 to first inject more urea into the exhaust gas. If this measure leads to an increase in the sensor resistance, the nitrogen monoxide NO may not yet have been completely converted to nitrogen N₂ and water H₂O. The amount of urea injected is now increased until the sensor resistance reaches the maximum value indicated by an arrow in FIG. 14 and that the catalyst 2 leaving exhaust gas contains neither nitrogen monoxide NO nor excess ammonia NH₃.

If the multiple injection of urea leads to a lower sensor resistance, the NH₃ excess in the exhaust gas must be reduced by reducing the amount of urea (see right part of the flowchart). In the diagram in FIG. 14, the maximum value of the sensor resistance defining the optimal amount of urea is therefore approached from the right.

The invention is of course not limited to the exemplary embodiments described. For example, it is possible to arrange a second detector based on sputtered Al₂O₃ / V₂O₅ layers in the exhaust tract in front of the injector 7. This detector is then preferably used to monitor the regulation described, by measuring the NO concentration and comparing it with the amount of urea injected in each case. The NH₃ sensitivity of the detector does not interfere, since the engine exhaust gas in front of the injector 7 contains no ammonia NH₃.

Instead of urea, ammonia in aqueous solution or gaseous ammonia can also be used as the reducing agent, the reducing agent also being able to be injected directly into the SCR catalytic converter 2.

The method according to the invention can of course also be used in so-called DeNO _x systems for smoke gas denitrification (see, for example, / 3 /).

Literature:

/ 1 / Motortechnische Zeitschrift 49 (1988) 1, pp. 17 to 21
/ 2 / Motortechnische Zeitschrift 54 (1993) 6, pp. 310 to 315
/ 3 / Environment, 1986 No. 1, Technical report flue gas cleaning, FR 19 to FR 25

Claims (9)

Method for reducing the nitrogen oxide concentration in the exhaust gas of an internal combustion engine or an incineration plant, in which an ammonia-containing reducing agent is added to the exhaust gas and in which nitrogen oxides are converted to nitrogen and water in a catalyst unit (2) through which the exhaust gas flows,
characterized, - That a first sensor element (12) responsive to both nitrogen monoxide and ammonia is arranged in the exhaust gas stream behind the catalyst unit (2), - That the resistance of the nitrogen monoxide and ammonia concentration or the electrical conductivity of the sensor element (12) are measured and - That the exhaust gas is added such an amount of the reducing agent in which the electrical resistance of the sensor element (12) is greatest or the electrical conductivity is the smallest. Method according to claim 1,
characterized,
that gaseous ammonia, ammonia in aqueous solution or urea is added as a reducing agent to the exhaust gas. The method of claim 1 or 2,
characterized,
that the electrical resistance or the electrical conductivity of a sensor element (12) consisting of a metal oxide-vanadium oxide mixture is measured. Method according to claim 3,
characterized,
that the electrical resistance or conductivity of an alumina-vanadium oxide mixture or an iron oxide-vanadium mixture existing sensor element (12) is measured. Method according to one of claims 1 to 4,
characterized,
that the electrical resistance or the electrical conductivity of a sensor element (12) consisting of a vanadate MeVO₄ is measured, where Me denotes a trivalent metal. Method according to claim 5,
characterized,
that the electrical resistance or the electrical conductivity of an aluminum or iron vanadate sensor element (12) is measured. Method according to one of claims 1 to 6,
characterized,
that a layered sensor element (12) contacted by a pair of electrodes (11, 11 ') is used. Method according to one of claims 1 to 7,
characterized,
that the sensor element (12) is actively heated and kept at a constant temperature. Method according to one of claims 1 to 8,
characterized,
that a second sensor element corresponding to the first sensor element (12) is arranged in the exhaust gas stream upstream of the catalyst unit (2) and used to measure the nitrogen monoxide concentration.

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